This invention relates to indexable cutting inserts and to a rotary cutter adapted to use such inserts. The invention has more particular reference to an indexable cutting insert which is of the "on-edge" type and which includes selectively useable cutting edges with each edge being adapted to cut at positive axial and radial rake angles.
On-edge inserts are disclosed in Hertel U.S. Pat. Nos. 3,142,110; 3,226,797 and 3,490,117 and in Erkfritz U.S. Pat. Nos. 3,694,876 and 3,762,005. The term "on-edge" applies to the presentation of the cutting material of the insert to the workpiece in such a manner that the compressive strength of the cutting material which opposes the tangential forces imposed on the insert is greater than the compressive strength which opposes the other forces exerted on the insert. Accordingly, an insert having a given volume of cutting material can be forced through the work with greater power without failing if the insert is of the on-edge type.
When the active cutting edge of the insert is disposed at positive axial and radial (i.e., double positive) rake angles, the edge is relatively sharp and less power is required to cut the workpiece than is the case when the edge is at a negative rake and is relatively blunt. Accordingly, an on-edge insert with cutting edges adapted to cut at double positive rake angles is advantageous in that the active cutting edge can cut the work with relatively low power and yet can withstand comparatively high forces before failing. Thus, the insert gives the user a choice of optimizing power and cutting speed according to his particular needs.
I have previously made an insert which is generally similar in shape to that disclosed in FIG. 4 of Stier U.S. Pat. No. 3,289,271 and have used that insert for on-edge cutting in a rotary cutter. For example, that insert can be employed in a face milling cutter. When so employed, however, it is necessary to position the insert at a lead angle on the cutter body in order to provide the necessary face clearance between the insert and the workpiece. When the insert is positioned at a lead angle, it is not capable of making a right angle cut and thus cannot be used for milling a workpiece which might have a square shoulder or the like.
Inserts of the type which I have previously made also may be placed in side-by-side relation in the rear stations of the angularly spaced slots of a helical end milling cutter. In such an arrangement, the inserts in one slot are staggered axially with respect to the inserts of an adjacent slot so that material which is left uncut between two side-by-side inserts in one slot is cut away by the insert of the adjacent slot. Such inserts, however, are not suitable for use in the front stations of the slots of an end mill since the inserts cannot be located to provide face clearance and are not capable of making a right angle cut.